An accurate knowledge of a bandwidth of an optical source such as a laser is important in many scientific and industrial applications, such as, for example, for critical dimension control in deep ultraviolet (DUV) semiconductor photolithography in which a substrate or wafer is irradiated by a light beam produced by the optical source while the substrate or wafer is moved axially along an optical axis of the light beam.
The bandwidth of laser light is the width of the intensity spectrum of the laser light output from the laser, and this width can be given in terms of wavelength or frequency of the laser light. Any suitable metric or mathematical construction related to the details of the optical source spectrum can be used to estimate the bandwidth of the laser light. For example, the full width of the spectrum at a fraction (X) of the maximum peak intensity (referred to as FWXM) can be used to estimate the laser light bandwidth. As another example, a width of the spectrum that contains a fraction (Y) of the integrated spectral intensity (referred to as EY) can be used to estimate the laser light bandwidth.